Cardiotherapeutic defibrillators, once used only by trained medical personnel, are now being made available for use by the general population, including individuals having little or no training. The defibrillators contemplated for general use are of the automatic external type and include on-board real time diagnostic capability to intervene or otherwise control the defibrillator therapy being administered. In general, the defibrillators deliver a relatively high voltage, low energy pulse or series of pulses to a patient suffering cardiac arrhythmias, such as ventricular fibrillation. The power supply relied upon to deliver the defibrillation therapy typically comprises one or more batteries carried on board the defibrillator unit or an electrical power utility supplying power to a building, for example. Because of the nature of the electrical therapy required, it is generally not possible in a practical device to supply the therapeutic energy upon instantaneous demand, by drawing from the power source. Instead, energy from the power source is typically accumulated over a period of time in one or more defibrillator capacitors that are later discharged to deliver the desired defibrillation therapy. It is usually critical that the defibrillation therapy be delivered as quickly as possible, given the nature of the medical threat encountered.
Practical defibrillation equipment must also be capable of rapid discharging (or disarming) in order to prepare for a controlled sequence of operation. Discharging may be required, for example, when a portable defibrillation unit is to be packed away for return transport to a hospital or dispatch office. At other times, discharging of a defibrillator capacitor bank is required when the therapeutic action is to be performed at a lower capacitor voltage. For example, patients of different ages require adjustments in the defibrillation voltage applied, with younger patients requiring lesser voltage levels. A patient's age may, for example, be indirectly conveyed to the defibrillation equipment by the choice of defibrillator paddles connected to the defibrillation equipment. Currently, sophisticated automated defibrillation units exist in the art that are capable of determining the defibrillation voltage required based on the size of the paddles selected. These units then automatically discharge the capacitor to the appropriate voltage setting. Alternatively, these units may require operator intervention to confirm the voltage setting. In any case, excess voltage stored in the capacitor is discharged to achieve the correct voltage level.
In other types of defibrillation equipment in common use today, an operator is required to manually select the defibrillation voltage, either directly or indirectly through settings bearing various legends. An inexperienced or untrained field operator could, by cycling the defibrillator voltage setting, cause the voltage reduction circuit undue stress. Typically, the greatest stress is borne by a discharge resistor or a like dissipative disarm device which can become extremely warm during this type of unusual operating condition. Unusually heavy use, even though otherwise proper, could also cause unacceptable stress on a defibrillator disarm circuit.
Previous disarm circuits used for discharging a capacitor bank have been constructed utilizing a separate current path outside of the energy switching components. This current path requires a high voltage switch, separate from the energy switching components to control the current flow through a discharging device. Alternatively, to avoid high voltage components, several smaller low voltage switches can be placed in series. In either case, a distinct and separate switch or switches in addition to those used for energy switching functions are required, undesirably increasing the component count and space requirements for the defibrillator unit.
For example, FIG. 1 shows a discharge circuit utilizing a temperature controller 109 that monitors the temperature, allowing a controller 108 to modulate the current through discharge component 103 by on-off modulation of a high voltage switch 105. However, this device requires the use of the high voltage switch 105, in addition to the high voltage switches present in the energy delivery circuit 102, thereby increasing the part count and space requirements of the defibrillator unit.
In addition, previous defibrillator output circuits fail to account for shoot-through current that can be generated in an energy delivery circuit configured as an H-Bridge, causing undesirable consequences. For example, referring to FIG. 2, a prior art defibrillator circuit is shown capable of delivering an energy pulse to a patient load 217. After capacitor 201 is charged to the desired voltage, resistors 215 and 216 hold the patient connections to a voltage potential that represents the positive potential across capacitor 201 when switches 211, 212 and 213 are in the off state. In this state, no current flows through the patient load 217. When current flow is required, switches 212 and 213 are rapidly turned on. The voltage to current transfer ratio or gain determines the gate voltage required to turn on the switches 212 and 213. The gain of the switch is a variable parameter and may not be tightly controlled. The rapid turn-on of switch 213 causes a large negative dv/dt (change in voltage over time) to be present at the emitter of switch 211. The gate of switch 211 should follow the negative voltage swing with current flowing through resistor 233 such that a potential difference between the gate and emitter of switch 211 remains below the turn-on threshold. However, parasitic capacitances, represented by capacitors 218 and 219, are formed by the internal capacitances of the switch itself, other component capacitances, and the electrical connections within the board layout. These parasitic capacitances form a capacitive voltage divider that can cause the voltage at the gate of switch 211 to build-up, thereby causing a partial or full nuisance turn-on of switch 211 while switch 213 is conducting. As can be seen in FIG. 2, if switches 211 and 213 are both conducting simultaneously, a direct path to ground is formed allowing a shoot-through current pulse to flow from capacitor 201. This current pulse can be of a large magnitude and possibly cause damage to switches 211 and 213, permanently damaging the defibrillator. The shoot-through turn on is mainly caused by the Miller capacitances of switch 211. These capacitances are hard to control, and are only approximated on the data sheets of the components.
In external defibrillator units, one common disadvantage is in their size. As these units are meant to be portable so that they can be easily transported by the end user during emergency situations, large and bulky units are undesirable. On the other hand, the compact size that is desired generally competes with the need to deliver relatively high energy levels to a patient for effective defibrillation. In smaller size units, the capacitors utilized for the energy storage device generally have to be sized smaller as well. The problem this creates is that the voltage developed in the unit has to be higher to deliver the required energy level to a patient, particularly for multiphasic energy pulses so that the energy is not exhausted prior to the desired termination of the delivered energy. To this end, the width or duration of the energy pulse generally has to be made shorter than is desirable in prior small defibrillator units.